An optical apparatus and method for the determination of impurity components present in both homogeneous and non-homogeneous light-scattering products with light sources having wavelengths in the visible and near-infrared regions has been disclosed in U.S. Pat. No. 4,755,048. The objective of that method is to measure the impurity level and product level in a light scattering material, with a requirement for the monitoring of three wavelengths for background, impurity and product. For the operation of Kaufman's apparatus and method, it is necessary to first characterize the component to be monitored such that a readily monitorable characteristic peak may be identified.
By contrast, this invention is a simpler technique, in which a product level is inferred by measuring only the concentration of a proportionally-added marker by the ratioing of two scattered wavelengths (one corresponding to the marker and one corresponding to the matrix). Even if the product has no readily monitorable peak, this invention will work. Moreover, for this invention, there is no requirement to determine a wavelength which is absorbed by components to be monitored, but rather only a minimal requirement to determine where the components to be monitored do not absorb.
Although Kaufman suggests the use of an optical marker when the product has no readily distinguishable absorption band, his method still involves a comparison at three wavelengths, that of the impurity, that of the product (as enhanced by the optical marker), and that of the background. The impurity is still directly monitored by this method. Thus, as will be described, this invention is more efficient than that disclosed by the Kaufman reference because there is no requirement that a product be directly monitored, nor is there a need to monitor three different wavelengths in order for the method to operate.